This invention pertains to mixed fertilizers, that is those providing primary nutrients in particulate form, and to the preparation of such fertilizers.
One of the disadvantages of commercially available fertilizers is that they do not compensate for the changes in soil acidity which are due to the fertilizer being applied. In solid fertilizers nitrogen has been derived from sodium and potassium nitrates. Now it is usually derived from ammonium salts, such as nitrates and phosphates, or from urea, which hydrolyzes to form ammonium carbonate. Plants can use ammoniacal nitrogen, but most of it is converted to the nitrate form of nitrogen by nitrifying bacteria. Therefore, it is usually the nitrate that is available for use by crops.
During the nitrification process when bacteria convert ammonium ions to nitrate ions, hydrogen ions are released. These hydrogen ions are attracted to and held by soil colloids. The more hydrogen ions involved in this exchange complex the greater the acidity of the soil. At that point nitrification slows down since nitrification rates are usually low in acid soils.
It will be appreciated that nitrogen compounds are a source of soil acidity. In addition nitrates are a major factor in the leaching of other nutrient salts such as calcium and magnesium and other cationic nutrients from soils. These bases are replaced by the available hydrogen ions, rendering the soil even more acidic.
It is to be understood that in addition to supplying nitrogen, phosphorous and potassium (N-P-K), a desirable fertilizer should offset soil acidity changes brought about by nitrification. Calcium carbonate accomplishes this. It reduces soil acidity through base exchange. The calcium ions of the calcium carbonate replace hydrogen ions in the soil, preliminarily forming H.sub.2 CO.sub.3. In solution the resulting carbonic acid gives off carbon dioxide, leaving water in the soil. As pointed out in U.S. Pat. No. 4,015,973 fertilizers have what is known as a "potential acidity in terms of calcium carbonate equivalent" which should be indicated on each bag of fertilizer. The object is to mix CaCO.sub.3 in the right proportions with the N-P-K fertilizer nutrients to neutralize the potential acidity thereof. By "fertilizer nutrients" we mean primary nutrients, although secondary nutrients other than calcium, along with micronutrients can be included. Unfortunately on a commercial scale this has not been accomplished.
If ground CaCO.sub.3 particles are too large the exposed surface area is insufficient for the desired chemical reactions. In the pulverized state of CaCO.sub.3 necessary for dissolution, because of its hygroscopic properties, a form of CaCO.sub.3 which is sufficiently stable in admixture with the fertilizer materials for commercial distribution has not been achieved.
In U.S. Pat. No. 4,015,973 a process is provided for making a pelletized limestone-bentonite soil neutralizer. It is asserted that the final product is not materially affected by exposure to the atmosphere. However, with the bentonite in matrix form throughout the pellets it is not possible to make a commercially successful product. If the clay is heated too high, approaching a ceramic, the entire pellet will be hard. Otherwise, on the absorption of water the swelling action of the clay causes the particles to crumble. In the formation of the granules in U.S. Pat. No. 4,015,973 the CaCO.sub.3 and bentonite are dry mixed. They are then pelletized using water as the binder. The result is a CaCO.sub.3 -bentonite matrix which is too soft for commercial acceptance. George G. Judd alludes to this in U.S. Pat. No. 4,410,350. Referring to U.S. Pat. No.4,015,973 Judd points out that no previously suggested fertilizer composition has included a filler which materially reduces the tendency of the active fertilizer ingredients to cake on standing in more or less humid conditions. Unless bentonite is heated to a hard clay it swells and cakes.
In U.S. Pat. No. 3,214,261 limestone is mixed with a non-phytotoxic binding agent. In a disc granulator water is added to activate the mix. Here again the non-phytotoxic binding agent is disseminated throughout the pellet. In U.S. Pat. No. 4,954,134 a lignosulfonate is employed. It bonds the particles as they are being agglomerated, and hence is embedded throughout as a continuous matrix.
The matrix binders of the prior art have, therefore, not been totally satisfactory. Judd (U.S. Pat. No. 4,410,350) solves this matrix problem by pelletizing with water and then forming a hard, outer skin around them. However, to accomplish this, Judd treats the pellets with a carbon dioxide-containing gas at a rate of fifty tons per hour at 680.degree. F. Herein the problem is solved without the high temperature gas treating equipment.